Multi-band radios rarely fail compliance because one block is outright broken; they fail because small, individually ‘acceptable’ leakages stack up. Spurious emissions budgeting is the discipline of deciding, early and quantitatively, how much unwanted energy each subsystem is allowed to contribute at the antenna port (and sometimes at conducted test points) so the assembled platform passes across every band, mode, temperature and production tolerance.
If you are the RF compliance lead on a multi-standard platform (LTE/NR, Wi-Fi, satcom links, GNSS, tactical waveforms), the hard bit is not knowing the limit lines. The hard bit is keeping the whole architecture honest when the PA wants efficiency, the LO wants phase noise margin, the digital team wants bandwidth, and the mechanical team wants one shared aperture.
Why spurious emissions budgeting is getting harder (not easier)
Three industry shifts are making spurious management more of a system exercise than a late-stage filter tweak.
- Regulators are leaning on harmonised limits and clearer test expectations. A good example is the FCC’s January 2024 proceedings around wireless multi-channel audio systems, which reference ETSI-style spurious limits (with different limits below 1 GHz, tighter constraints in certain protected bands, and a different limit above 1 GHz) and highlight that measurement procedure details matter. The lesson for multi-band platforms is simple: you can meet a number on paper and still lose time in the chamber if your measurement setup, detector choice, or RBW/VBW assumptions are not aligned with the target standard.
- 3GPP keeps tightening the relationship between out-of-band emissions and the spurious domain. In NR, adjacent channel leakage ratio (ACLR) and spectrum emission mask (SEM) deal with the near offsets, while spurious is what remains beyond the defined out-of-band boundary (commonly tied to channel bandwidth plus an offset). That boundary definition forces you to treat filtering, linearity and LO-related products as a continuous emission-control problem, not separate ‘ACLR’ and ‘harmonics’ workstreams.
- Spectrum sharing pressure is rising. WRC-23 outcomes and national spectrum strategies continue to push IMT and other services into bands with incumbents (satellite and radar are common neighbours). In shared or adjacent environments, spurious products that were once ‘nice-to-have’ reductions become coexistence enablers, especially when the victim is a sensitive receiver with a low noise floor and long integration time.
Define the problem properly: what counts as spurious (and where it is measured)
Before you can budget, you must lock the compliance interpretation:
- Domain boundaries: Separate in-band, out-of-band (OOB) and spurious domains per the relevant standard(s). For cellular, keep the ACLR/SEM domain distinct from the spurious domain. For licence-exempt and bespoke defence radios, the domain boundaries may be defined by the local regulatory mask plus additional programme requirements (for example, platform self-compatibility constraints).
- Reference point: Is the limit at the antenna port (EIRP/ERP), a conducted port (dBm into 50 Ω), or a radiated measurement with specified distance and antenna factor? Multi-band platforms often mix these depending on the region and band.
- Detector and bandwidth: Average vs peak, and the mandated measurement bandwidths (RBW) that apply at different frequency ranges. This is where budgeting often goes wrong: teams allocate ‘dBc’ budgets without mapping them to the actual compliance measurement conditions.
- Operating modes: Highest duty cycle, worst-case modulation, aggregated carriers, and any simultaneous transmit scenarios (including ‘unrelated’ radios that share power, clocks, or an aperture).
Spurious emissions budgeting: a system-level method that actually survives integration
A practical budget needs to be traceable from the regulatory limit back to design knobs. The approach below is what tends to survive real integration programmes.
1) Start from the limit line at the antenna, then work backwards with margins
For each band and each critical frequency region (fundamental band edges, 2nd/3rd harmonics, known sensitive neighbour bands), define:
Allowed spurious at antenna (dBm or dBc) = Regulatory limit − measurement uncertainty margin − production spread margin − temperature/voltage margin − integration unknowns
Be honest with the margins. If you have not characterised PCB-to-PCB spread, or you are reusing an LO across bands, keep margin until proven otherwise.
2) Allocate the budget by mechanism, not by organisational ownership
‘PA team gets 10 dB, synthesiser team gets 10 dB’ sounds tidy, but it is rarely correct. Allocate by emission mechanism because mechanisms combine non-linearly:
- Harmonics: PA non-linearity, output matching, and any frequency multiplication. Harmonics are typically filterable, but only if your filter has the right stopband at the right impedance and temperature range.
- Broadband noise skirts: PA noise, DAC/ADC noise, and PLL phase noise translated through upconversion. These are harder to notch-filter without impacting group delay or EVM.
- Discrete spurs: PLL reference spurs, fractional-N spurs, DC-DC converter switching products, digital interface clocks, and intermodulation products created by simultaneous transmitters or leakage into non-linear junctions.
- Once mechanisms are clear, you can set budgets like ‘reference spur at ±19.2 MHz offsets must be below X dBc in conducted, assuming Y dB post-PA gain and Z dB filter rejection’.
3) Convert every budget item into the same units and the same measurement reality
Compliance is typically in absolute power per measurement bandwidth. Designers often think in dBc. Convert early and include the correct RBW:
dBm in RBW = Carrier power (dBm) − spur level (dBc) + 10·log10(RBW normalisation)
If your spur spec is measured in, say, 1 kHz but compliance is 100 kHz, you cannot ignore the difference for noise-like terms (and you must be careful not to apply noise scaling to a truly discrete spur).
Common multi-band failure modes (and how to budget against them)
Most spurious surprises in multi-band platforms fall into a few patterns.
Shared LO and clocking: the ‘one reference to rule them all’ trap
A shared reference simplifies BOM and synchronisation, but it couples spurs across every RF chain. A weak reference spur that is harmless in Band A can land in a protected band when you tune Band B. Budget explicitly for reference-related products across all tuning plans, not just the nominal deployment band.
Simultaneous transmit intermodulation: the platform creates its own victim
Two transmitters sharing a power rail, enclosure, or antenna switch can generate intermods in unexpected places. If your platform can transmit LTE/NR while maintaining a satcom uplink, treat intermod products as first-class budget items. The victim may be your own GNSS, a telemetry receiver, or an adjacent radio in the same product.
Filter ‘specmanship’: stopband looks great on paper, then collapses in-circuit
Datasheet rejection is often quoted at 50 Ω with clean fixtures. In real-world radio, the PA presents a frequency-dependent impedance, the switch introduces parasitics, and the filter may generate heat. Budget with in-situ S-parameters or, at a minimum, add a margin for mismatch and temperature drift.
A practical spurious emissions budget template for compliance leads
If you need something you can run in a spreadsheet and defend in a design review, use a table per band with columns like:
- Frequency region: (e.g., 2nd harmonic, 3rd harmonic, known neighbour band, LO feedthrough region)
- Limit and condition: (absolute dBm/ERP/EIRP, RBW, detector, average/peak)
- Allowed at antenna: after all margins
- Contributions (in dBm): PA harmonics, mixer products, PLL spurs, DAC images, DC-DC spur feedthrough, intermod from concurrent TX
- Mitigations assigned: filter stopband target, PA linearity target, reference filtering, shielding, PCB partitioning, power integrity actions
- Verification method: conducted bench, radiated pre-scan, chamber test, temperature sweep, worst-case tune list
The key is to make every row testable before final certification. If a budget line cannot be verified until the full platform is built, it is not a budget; it is hope.
Where Novocomms Space expertise fits: when the RF platform has to coexist, not just comply
Space and aerospace programmes are unforgiving teachers: you do not get to ‘patch’ an on-orbit payload, and spectrum coexistence is often as critical as link budget. That experience translates directly to multi-band terrestrial and hybrid platforms.
At Novocomms Space, we support programmes where spurious control is engineered as part of the RF chain from day one: front-end architectures, filtering strategies, antenna integration, and RF system-level verification. Typical use-cases include:
- Multi-function apertures: when one antenna system must support several bands without letting one transmitter pollute another receiver path.
- Integrated RF payload building blocks: where careful selection and placement of filters, LNAs, and switching reduces the risk that a ‘minor’ spur becomes a mission-limiting interferer.
- Pre-compliance and qualification-style testing: structured sweeps, worst-case tune lists, and margin tracking so compliance is a managed outcome, not a late surprise.
Conclusion: treat spurious as a budgeted resource, not a post-layout firefight
On a multi-band radio platform, spurious performance is not owned by the PA, the synthesiser, or the filter. It is a system property shaped by architecture choices, tuning plans, power integrity, shielding, and how you interpret and test to the standard. The teams that pass first time are the ones that convert limits into a living spurious emissions budget, allocated by mechanism, and verified early with the same bandwidths and detectors the compliance lab will use.
If you are building a multi-standard platform and want a second set of eyes on your spurious emissions budget, architecture, or test plan, speak to Novocomms. Contact us here: https://novocomms.com/contact-us/.
